1. Field of the Invention
This invention relates generally to a compression driver, phasing plug and an assembly of a compression driver phasing plug having a tight dimensional tolerance.
2. Related Art
A compression driver typically comprises a pole piece made of ferromagnetic material having a magnetic air gap to receive a voice coil. The exit or opening of the compression driver is adaptable for coupling to the throat of a horn. A diaphragm, usually circular with a central dome-shaped portion, is mounted adjacent the rear opening of the bore to allow the diaphragm to freely vibrate. Attached to the edge of the diaphragm's dome is a cylindrical coil of wire, the voice coil, oriented so that the cylindrical axis of the coil is perpendicular to the diaphragm and coincident with the axis of the pole piece bore. A static magnetic field, usually produced by a permanent magnet, is applied so that an alternating signal current flowing through the voice coil causes it to vibrate along its cylindrical axis. This in turn causes the diaphragm to vibrate along the axis of the bore and generate sound waves corresponding to the signal current. The sound waves are directed through the bore toward its front opening.
The front opening of the bore is usually coupled to the throat of a horn, which then radiates the sound waves into the air. In the description that follows, the term “throat” is used to mean either downstream end or exiting end of the pole piece bore or the actual entrance of a horn. Interposed between the diaphragm and the pole piece bore is a perforated structure known as a phasing plug for impedance matching the output of the diaphragm to the horn. Within the phasing plug are one or more air passages or channels for transmission of the sound waves. The surface of the phasing plug adjacent to the diaphragm corresponds spherically and is positioned fairly close to the diaphragm while still leaving an air gap, or compression region, in which the diaphragm can vibrate freely.
The phasing plug performs two basic functions. First, because the cross-sectional area of the air channel inlets are smaller than the area of the diaphragm, the air between the diaphragm and the phasing plug (i.e., the compression region) can be compressed to relatively high pressures by motion of the diaphragm. This is what allows a compression driver to output sound at greater pressure levels than conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the loudspeaker is thus increased by virtue of the phasing plug being placed in close opposition to the diaphragm to minimize the volume of air between the diaphragm and the phasing plug. Second, as the name “phasing plug” implies, the path lengths of the air channels within the phasing plug may be equalized so as to bring all portions of the transmitted sound wave into phase coherence when they reach the throat. Without such path length equalization, sound waves emanating from different air channels would constructively or destructively interfere with one another at certain frequencies so as to distort the overall frequency response.
Manufacturing the compressor driver phasing plug, however, can be a time consuming and expensive process. For example, to make a compression driver and phasing plug, a number of parts need to be assembled either by gluing or press-fitting the parts together, and then the assembly is machined for finishing. Unfortunately, the labor intensive process of assembling the number of parts adds cost to the manufacturing process. Moreover, the tight dimensional tolerances that must be kept are difficult to achieve. That is, because of the inherent variances that exist in casting each part, when they are combined, the size of the air passages or channels may vary, i.e., one air passage may be smaller or larger than the specification requires, so that there is distortion in the frequency response. Therefore, there is still a need to manufacture a compression driver phasing plug that is easy to manufacture yet with tight dimensional tolerances.